| Surface-enhanced Raman Scattering(SERS)is an analytical method to obtain molecular vibration fingerprint information,which has been widely used in material science,biomedicine and food safety.The SERS signal enhancement come from the plasmon of precious metals.When the frequency of incident light matches the frequency of the plasmon oscillation,the collective oscillation of electrons in confined space will produce enhanced local electric field,which is also called"hot spots".The intrinsic Raman signals of most substances are extremely weak.Therefore,researchers constantly pursue for reliable SERS substrates with rich hot spots,and have prepared a series of sensitive SERS substrates by chemical synthesis and physical etching.Single molecule SERS can even be realized through the coupling of plasmon between nanoparticles.However,experimental results have shown the quantum tunneling effect in a sub-0.5 nm nanocavities will lead to charge transfer between metal interfaces,quenching the extremely strong local electric field.In addition,it is difficult to accurately place molecules in such a highly localized hot spot.Therefore,there are certain limitations to improve SERS sensitivity by reducing the spatial volume of electrons oscillation.An additional enhancement mechanism should be considered to further enhance the local electric field of the plasmon.SERS can be understood as an excitation process and an emission process:The incident light firstly excites the plasmon,and the scattering cross sections of the molecules are increased.The scattering light of the molecules is then amplified by plasmon and emitted to the far field.In most of existing works,the incident light directly excites plasmon which is the connection of the excitation and emission processes.However,the optimization of the light field before exciting plasmon is usually ignored,which may be one of the most effective ways to improve the intensity of plasmon.For optimizing the SERS excitation process,the thesis is based on evanescent field on the surface of planar dielectric waveguide,to design and construct evanescent field-excited plasmon SERS substrate.The properties and excitation method of SERS substrate are studied by constructing nano-hemisphere,nano-cone and nano-gap arrays in the evanescent field.In addition,a highly sensitive non-labeled SERS method for cell membrane detection is developed based on the principle of evanescent field-excited plasmon.The main contents includes following three aspects:(1)Investigate the properties of evanescent field-excited plasmon under polarized light.Traditional metal film-based plasmon can only be excited by p-polarized light,while the waveguide mode can be excited by both p-polarized and s-polarized light.A gap-type antenna array and a tip-type antenna array are designed on the waveguide surface for transverse electric field and transverse magnetic field modes,respectively.As the results show,for the nano-hemispheres in transverse electric field mode,reducing the gap under a constant array period will change the waveguide mode,and the electric field will be confined in the waveguide layer,which will weaken the evanescent field at the interface.For the nano-cone in transverse magnetic field mode,the maximum electric field is located at the tip,while it can be changed to the bottom of nano-cone in the transverse electric field mode.This indicates that the hot position of nano-cone can be adjusted flexibly by adjusting the polarization of incident light,which is conducive to the label-free sensing and detection of biomacromolecules.The lowest detectable concentration of4-mercapto-benzoic acid can reach to 1.0×10-10 M.(2)Propose a SERS substrate based on plasmon near field-excited waveguide mode.Experiment and theory simulation proves when the incident light directly illuminates on the waveguide with a 8 nm-gap dimer array,the local field at gap will diffuse into the waveguide,and stimulate the waveguide mode.Finally,the local electric field enhancement of the nanometer gap array is realized under the dual excitation of evanescent field and incident light.The enhancement provided by the waveguide is equivalent to reducing the 8 nm gap between the dimers to 5 nm.The SERS substrate doesn?t use the necessary optical coupler(such as prism and high numerical aperture lens)for the traditional excitation waveguide mode.Thus,the detection structure of evanescent field-excited plasmon was simplified based on the near field excitation mechanism.(3)Explore the application of evanescent field-excited plasmon SERS substrate in cell membrane detection.The silver nanoparticles are tightly encased by cell membrane and excited by evanescent field to improve the detection sensitivity,which is expected to achieve label-free SERS characterization of cell membrane.A simulated cell membrane model composed of cardiolipin and cytochrome c is designed to verify the sensitivity of SERS substrate.It is proved that the detection limit of the SERS substrate for cytochrome c is 2.0×10-6 M by changing the concentration of cytochrome c loaded on the simulated membrane.The interaction of epidermal growth factor receptor with monoclonal antibody and epidermal growth factor on real cell membrane are detected,respectively.Through the changes of the label-free SERS signals of cell membrane,the identified site of epidermal growth factor receptor is considered to be related to phenylalanine and proline,respectively.The evanescent field-excited plasmon SERS substrate will provide new opportunities to analyze the physiological process of membrane proteins at the molecular level. |
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